1. Field of the Invention
The present invention relates to an active power condition for AC load characteristics. Particularly, the present invention relates to the active power conditioner connected in parallel with the AC loads that are non-linear or contain-reactive power. More particularly, the present invention relates to the active power conditioner applied to adjust the AC loads for performing a linear resistance characteristic so as to reduce harmonic and reactive power currents of the AC loads.
2. Description of the Related Art
Recently, a variety of non-linear loads such as rectifiers, battery chargers, motor drivers and UPS (Uninterruptible Power Supply) have been widely used in a distribution power system. These non-linear loads can generate a great amount of harmonic currents injecting into the distribution power system. This may result in overheating transformers, fluctuations of rotary electric machines, voltage distortion of utility power and damage to electric equipment in the distribution power system.
In order to improve the problems with the harmonic pollution, many harmonic control standards, such as IEEE519-1992, IEC1000-3-2, and IEC1000-3-4 etc., have been established. Many countries have therefore enforced these control standards. There is also established a power control provisional standard for harmonic by Taiwan Power Company in Taiwan. Therefore, how to solve the harmonic problems is an important topic in today's power system worldwide.
On the other hand, many of the loads in the distribution power system are inductive (inductor-type) loads that result in lagging power factor of reactive power. To compensate the lagging reactive power, the distribution power system must further supply reactive power to the loads in addition to real power. Accordingly, it would be disadvantageous that the efficiency of the distribution power system is lowered and the voltage regulation in the load side is poor. Furthermore, it would be also disadvantageous that a larger capacity of the power transmission of the distribution power system is required. In Taiwan, Taiwan Power Company has also urged to reduce the reactive power generated from the users existing in the distribution power system.
As has been explained above, linear loads with a resistance characteristic are preferred in the distribution power system. However, the linear resistance loads only consume real power in the distribution power system and may not produce harmonics. Traditionally, passive electric components are used to form a power conditioner applied in the distribution power system so as to operate the loads for creating a linear resistance characteristic. For example, a passive power filter can reduce harmonic currents produced by the non-linear loads, and improve power factors. A power capacitor set is also used to reduce reactive currents produced by inductive loads. However, there exist some problems with using the passive power conditioner applied to create the linear resistance characteristic. For example, the passive power filter may cause drawbacks of serial/parallel resonance, injection of neighboring harmonic and lower filtering effect; and the power capacitor may also cause drawbacks of serial/parallel resonance, injection of neighboring harmonic, unvaried reactive power compensation and incapability of linear adjustment.
In order to solve the problems with the passive power conditioner for the AC load, the active power conditioner comprising power electronic components, as shown in FIG. 1, has been developed. Traditionally, the active power conditioner (so-called active power filter) includes a DC power capacitor 91, a power converter 92 and a high-frequency filtering circuit 93. In this case, the active power conditioner electrically connects with a load 2 in parallel and operates therewith. A non-linear characteristic of the load 2 can be adjusted and shifted to a linear resistance characteristic by the active power conditioner which can eliminate reactive current and harmonic currents of the load 2. Accordingly, a sinusoidal waveform of a current supplied from a power source 1 has phase identical with those of voltages of the power source 1. Consequently, the characteristic of the load 2 can be conditioned to be a linear resistance characteristic.
An active power conditioner for adjusting AC loads, described in U.S. Pat. Nos. 5,321,598, 5,614,770 and 5,977,660, includes a DC power capacitor, a power converter, a high-frequency filter and a control circuit. In operation, the active power conditioner detects a load current, a power source voltage and a DC capacitor voltage that are calculated in a complicated process to generate a compensation current signal for sending to a feedback controller. An output current of the power converter is further detected and sent to the feedback controller. The feedback controller processes the compensation current signal and the output current of the power converter by means of a closed-loop control. Accordingly, the output current of the power converter can be adjusted to respond to changes of the compensation current signal. Although it would be advantageous that the active power conditioner is, however, successful in reducing harmonics and creating a linear resistance characteristic, constructions of the control circuit are complicated. Hence, there is a need for improving the active power conditioner for the AC loads.
As is described in greater detail below, the present invention intends to provide an active power conditioner for AC load characteristics. This active power conditioner can generate a driving signal for a power converter by detecting a voltage of a DC power capacitor for processing a closed-loop control and detecting a current of a power source for processing a feed forward control. In the present invention, calculation of the compensation current signal and detection of the output current of the power converter are not required. In this manner, the active power conditioner can adjust a non-linear load to be performed a linear resistance characteristic, reduce the harmonic and reactive current and simplify the control circuit.